1. Field of the Invention
The present invention relates to a speaker apparatus that emits sound to a limited spatial area.
2. Description of the Related Art
In recent years, speakers utilizing, for example, a parametric array effect have been known to be able to emit sound to a limited spatial area, that is, to have what is called a narrow directivity (see Non-Patent Document 1). The parametric array effect occurs as follows. For example, when an ultrasonic vibrator is used to emit an ultrasonic wave having its amplitude modulated by an audible area signal, the nonlinear sound propagation characteristic of sound wave in the air causes a modulation signal to be self-demodulated along an ultrasonic sound propagation path. Demodulated areas are distributed in an array in a direction in which the ultrasonic wave progresses. Accordingly, a demodulation wave has a high directivity in the progressing direction of the sound wave.
However, a problem with the use of the parametric array effect is the adverse effect of the powerful ultrasonic wave used on human bodies and the like.
FIG. 1 is a schematic sectional view of a speaker conventionally commonly used.
A common speaker 100 shown in FIG. 1 is mainly composed of a sound processing section 110 having an electroacoustic transducer 111 with a diaphragm 111a that vibrates on the basis of a signal and a case 112 that contains, for example, the electroacoustic transducer 111 and an amplifier (not shown) that amplifies the amplitude of a signal input.
The directivity of a sound emitted by a diaphragm of an electroacoustic transducer such as the one shown in FIG. 1 is generally determined by the wavelength λ of sound emitted by the diaphragm and the diameter of the diaphragm (the diameter of the diaphragm will be referred to as D). What is called an effective directional angle Φ (degree) at which a sound pressure of 1/√2 of its maximum value is approximately given by the following Expression 1:Φ≦29×λ/D   (Expression 1)
Accordingly, to improve the directivity of a sound emitted by the common speaker 100, shown in FIG. 1, it is necessary to increase relatively the diameter D of the diaphragm 111a of the sound processing section 110 relative to the wavelength λ of the sound emitted as shown in the above expression.
[Non-Patent Document 1] The Acoustical Society of Japan, “Dictionary of Acoustic Terms”, CORONA PUBLISHING CO., LTD., issued on Apr. 20, 1988, Page 479
However, actually, as the diameter of the diaphragm increases, more complicated facilities are disadvantageously required to precisely vibrate the diaphragm.